A. Technical Field
The present invention relates to: a thiol carboxylate ester which is excellent in polymerizability and in stability even if stored for a long term; and a process for stabilizing a thiol carboxylate ester.
B. Background Art
As to high refractive materials, inorganic optical materials such as glass are recently becoming replaced with high transparent synthetic resins. So they are, particularly, in the use for such as contact lenses. The high refractive resins are good in their light weight property, impact resistance, molding-processibility, and dyeability, therefore their field of application as optical plastic materials is extending.
However, dialkylene glycol bis(ally carbonate) resins, which are widely used as the optical plastic materials, have an insufficient refractive index of 1.49 to 1.50. Therefore, optical lenses obtained from these resins have a demerit in that the thicknesses of their centers and edges are larger than those of inorganic optical lenses.
In order to solve such problems, the present applicant developed a novel thiol carboxylate ester as shown by general formula (I) below (JP-A-162671/1988): (wherein: R denotes H or —CH3; and X denotes —CH2CH2SCH2CH2—, —CH2CH2OCH2CH2—, or —CH2CH2—).
This substance is a monomer which gives a resin having so high a refractive index and so excellent transparency as to be useful as an optical plastic material.
However, thereafter, it has turned out that: this monomer may have problems in point of its polymerizability, and further, when this monomer is stored for a long term, its stability, particularly, coloring, may be problematic, specifically, depending on conditions for polymerization or use of this monomer, its slightly low polymerizability or its stability during its long-term storage may be problematic. If the polymerizability of the monomer is slightly low, there is a concern about that there may occur problems in point of such as processibility (e.g. cutting property) and mechanical properties of a molded structure obtained by polymerization of the monomer. In addition, in the case where the monomer has colored as a result of the long-term storage, there is a concern about that, when this monomer is polymerized and then used as an optical plastic material, its transparency and refractive index may be low.
JP-A-199963/1994 discloses a process for polymerizing a thiol carboxylate ester containing a Michael addition product, but is silent about that the thiol carboxylate ester used as a monomer may have problems in point of its polymerizability and stability.
Incidentally, unless otherwise noted, the polymerizability, as herein referred to, means the polymerizability of the monomer in its polymerization reaction which is carried out in such as the resin-forming step (polymerization step). In addition, the stability as herein referred to is a performance concerning the quality of the monomer such as coloring inhibitability and storage stability.